LED lamps, which utilize light-emitting diodes, are used in many displaying elements of instruments such as mobile devices, PC peripheral equipments, OA equipments, various kinds of switches, light sources for backlighting, and indicating boards. The LED lamps are strongly required not only to have high efficiencies, but also to be excellent in color rendition when used for general lighting or to deliver wide color gamuts when used for backlighting. In order to enhance the efficiencies, it is necessary to improve those of fluorescent substances used therein. On the other hand, for the purpose of realizing high color rendition or a wide color gamut, it is effective to adopt a fluorescent substance emitting green luminescence under excitation by blue light.
Meanwhile, high load LED lamps generate heat in operation so that fluorescent substances used therein are generally heated to about 100° C. to 200° C. When thus heated, the fluorescent substances generally lose emission intensity. Accordingly, it is desired that the fluorescent substances less undergo the decrease of emission intensity even if the temperature rises considerably.
From the above point of view, an Eu-activated β-SiAlON phosphor (for example, disclosed in Patent document 1) can be regarded as an example of the fluorescent substances which emit green luminescence under excitation by blue light and hence which are suitably used for the aforementioned LED lamps. This phosphor is said to be very efficient when excited at 450 nm, and specifically its absorption ratio, inner quantum efficiency and emission efficiency under the excitation at 450 nm are approx. 65%, 53% and 35%, respectively.
In the meantime, a lot of energetic work has recently been devoted to development of flat panel displays, and the development has put emphasis on the study of PDPs (plasma displays) and LCDs (liquid crystal displays). However, field emission displays are expected to give much clearer images and hence to be more advantageous than the PDPs and LCDs.
The field emission display comprises a screen on which red, green and blue fluorescent substances are arranged, and also comprises a cathode spaced apart from and facing to the screen. The space between the screen and the cathode is smaller than that in a CRT. The cathode includes plural emitter elements as electron sources, which emit electrons in accordance with potential difference between the emitter elements and gate electrodes placed nearby. The electrons thus emitted are accelerated by an anode voltage (accelerating voltage) applied on the fluorescent substance side, and then made to impinge against the fluorescent substances, so that the fluorescent substances give off luminescence to display a clear image.
As for the fluorescent substances used in the field emission display constituted as described above, it is required to have sufficiently high emission efficiency and to keep exhibiting the sufficiently high emission efficiency even when saturation is achieved by excitation with a high current density. In view of only this requirement, sulfide phosphors (e.g., ZnS:Cu, ZnS:Ag), which are conventionally used as fluorescent substances for CRTs, may be considered as prospective candidates for the fluorescent substances usable in the field emission display. However, the sulfide phosphors such as ZnS are reported to decompose under the condition where a low-energy cathode ray display screen is excited. This is a serious problem because the decomposed products thus generated badly degrade a heat filament, from which electron beams are emitted. Further, conventionally used ZnS-based blue fluorescent substances are more liable to deteriorate in luminance as compared with red and green fluorescent substances. This raises another problem in that colors in displayed color images fade or change gradually with time.
The present inventors researched a SiAlON oxy-nitride phosphor capable of emitting blue luminescence (Patent document 2). This fluorescent substance less undergoes the decrease of emission intensity even if the temperature rises, as compared with conventional blue light-emitting fluorescent substances. Even so, however, it is still desired to develop a fluorescent substance having characteristics better than that phosphor. In addition, with respect to the fluorescent substances emitting light of colors other than blue, it is desired to improve their deterioration over time.